In signal processing of an imaging system such as a current digital still camera or a current video camera, such processing as gradation-converting an image signal is often performed. As one method of the gradation conversion processing, there is a method of calculating a gradation conversion characteristic using a histogram of an image signal. For example, in JP2007-124453A, a feature of contents is determined based upon information obtained from a histogram of an inputted image signal, so that contrast adjustment according to a feature of contents is made possible.